This invention relates to a flexible elastomeric cover for an electrolytic halide cell. More particularly, this invention relates to covers for mercury-type electrolytic chlorine cells having improved resistance to the permeation and penetration of deleterious fluids or gases.
Chlorine may be prepared by a continuous electrolytic process within an electrolytic cell, for example, of the mercury type as described in the Encyclopedia of Chemical Technology, Second Edition, Vol. 1, Pages 688-695 (1963). One such cell referred to as the DeNora cell is disclosed in U.S. Pat. No. 2,958,635 issued on Nov. 1, 1960 to V. DeNora. In this cell, graphite anodes and a liquid mercury cathode are suspended in a water solution of the metallic salt of chlorine such as sodium chloride after which a high current is passed through the brine or electrolyte solution. The ionized halogen migrates to the anode where two atoms combine to make a molecule which is discharged from the salt solution and recovered through a vacuum line. The free metal dissolves in the mercury cathode to form an amalgam which floats on the mercury and can therefore be withdrawn from the cell.
The free chlorine will normally contain a quantity of water from the brine which results in the formation of very corrosive substances in the hot gaseous chlorine atmosphere within the cell. Because very high electrical currents are necessary in the electrolytic process for producing chlorine, the atmosphere about the cell contains a high concentration of oxygen and ozone which especially when taken in combination, are extremely deleterious to most elastomeric compositions. Thus, electrolytic cell covers are subject to attack from both within and outside of the mercury cell.
Previously, these covers had been produced from laminates of elastomeric material with separate discernible layers integrally joined together during vulcanization. For example, one common laminate has been an inwardly facing layer of polyiosprene and an outwardly facing layer of neoprene rubber. In addition, U.S. Pat. No. 2,998,374 issued on Aug. 29, 1961 to P. G. Granfors discloses an elastomeric laminate having 3 layers or plies in which a layer of polyisoprene is sandwiched between inner and outer layers of neoprene rubber. It is also disclosed in this patent that chlorosulfonated polyethylene may be substituted for the neoprene and butyl rubber may be substituted for the polyisoprene. U.S. Pat. No. 3,450,621 issued on June 17, 1969 to R. F. Anderson discloses a laminated cover for a DeNora type cell characterized by an inner layer of natural rubber facing inwardly of the cell and an outer layer of ethylene propylene terpolymer material facing outwardly of the cell which may be bonded together by a tie gum layer of chlorinated butyl rubber disposed between the first named layers. These prior art constructions have not proven to be entirely satisfactory and have not successfully withstood the combined effects of hot wet chlorine gases within the cell and high oxygen and ozone concentrations immediately outside the cell.
The apparent rationale of previous chlorine cell cover constructions was that a layer of a chlorine impermeable polymer such as polyisoprene, e.g., natural rubber, was disposed at or near the inner surface of the cover to contact the corrosive chlorine atmosphere within the tank and the outwardly disposed layer of the cover was comprised of a highly ozone-resistant material such as neoprene rubber or ethylene propylene terpolymer rubber. Although the rationale seems logical enough in theory, it is apparently incorrect since under actual conditions, the cell covers as previously described have not withstood the combined effects of the substances within and outside the cells. For example, it has been found that the ozone resistant outer layers have shown as much effects of the chlorine degradation as have the inner layers of the cover and in fact have also shown failures which appear to be ozone cracking. On the other hand, laboratory data indicates that the cell covers should have the capabilities of withstanding high ozone concentrations for almost an indefinite period when material such as ethylene propylene terpolymer are used to form the outer layer of the cover. It has also been determined that the outer neoprene layers have not been sufficiently resistant to heat and high ozone concentrations.
U.S. Pat. No. 3,794,577, issued on Feb. 26, 1974 to B. H. Oliver and H. S. Custer (the latter being one of the present inventors), discloses an improved cell cover provided from blended compositions of single layers or combined laminated layers of heat and ozone resistant fluid impermeable materials. The cover is comprised of a vulcanized blend of polyisoprene resistant to hot wet chlorine gases and other corrosive fluid substances within the cell and butyl, chlorobutyl, bromobutyl rubber or chlorosulfonated polyethylene which resist the permeation or penetration of oxygen and ozone gases from outside the cell. Although these covers have proven satisfactory for long periods of service in chlorine cells, it is desired to produce covers which will perform satisfactorily for even longer periods.
In accordance with the present invention, an improved cell cover having increased resistance to the permeation and penentration of deleterious fluids from both within and without the cell is described hereinafter.